Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, additional metallic components have been added to platinum as promotors to further improve the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, tin, and the like. Some catalysts possess superior activity, or selectivity, or both, as contrasted with other catalysts. Platinum-rhenium catalysts by way of example possess admirable selectivity as contrasted with platinum catalysts, selectivity being defined as the ability of the catalyst to produce high yields of C.sub.5 + liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., methane and other gaseous hydrocarbons, and coke.
In a conventional process, a series of reactors constitute the heart of the reforming unit. Each reforming reactor is generally provided with a fixed bed, or beds, of the catalyst which receive downflow feed, and each is provided with a preheater or interstage heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or recycle hydrogen gas, is concurrently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, and a vaporous effluent. The former is recovered as a C.sub.5 + liquid product. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated and recycled to the process to minimize coke production.
The activity of the catalyst gradually declines due to the build-up of coke. Coke formation is believed to result from the deposition of coke precursors such as anthracene, coronene, ovalene and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation, the temperature of the process is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processes of this type the catalyst must necessarily be periodically regenerated by burning off the coke at controlled conditions.
Two major types of reforming are broadly practiced in the multi reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, ie., burning the coke from the catalyst. Reactivation of the catalyst is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In this type of operation, the reactors remain on stream for very long periods prior to catalyst regeneration, and reactivation, usually several hundred hours or more, e.g., at least about 700 hours, more often 2200 hours, or more. In the second, or cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series. Whereas the individual reactors remain on stream for periods of different length depending on their position in the series, rarely if ever do any remain on stream for as long as 200 hours prior to catalyst regeneration, and reactivation. In some reforming units, features of the semi-regenerative operation are found in conjunction with cyclic operations. These operations, termed "semi-cyclic," also necessitate that some of the reactors remain on-oil for long periods, typically from 700 hours to 1400 hours, prior to regeneration and reactivation of the catalyst.
Various improvements have been made in such processes to improve the performance of reforming catalysts in order to reduce capital investment or improve C.sub.5 + liquid yields while improving the octane quality of naphthas and straight run gasolines. New catalysts have been developed, old catalysts have been modified, and process conditions have been altered in attempts to optimize the catalytic contribution of each charge of catalyst relative to a selected performance objective. Nonetheless, while any good commercial reforming catalyst must possess good activity, activity maintenance and selectivity to some degree, no catalyst can possess even one, much less all of these properties to the ultimate degree. Thus, one catalyst may possess relatively high activity, and relatively low selectivity and vice versa. Another may possess good selectivity, but its selectivity may be relatively low as regards another catalyst. Platinum-rhenium catalysts, among the handful of successful commercially known catalysts, maintain a rank of eminence as regards their selectivity; and they have good activity. Nonetheless, the existing world-wide shortage in the supply of high octane naphtha persists and there is little likelihood that this shortage will soon be in balance with demand. Consequently, a relatively small increase in the C.sub.5 + liquid yield can represent a large credit in a commercial reforming operation.
Variations have been made in the amount, and kinds of catalysts charged to the different reforming reactors of a series to modify or change the nature of the product, or to improve C.sub.5 + liquid yield. Different catalysts, with differing catalytic metal components, have also been used in the different reactors of a series. The concentrations of the catalytic metal components on catalysts containing qualitatively the same metals have also been varied to provide progressively increasing, or decreasing, catalytic metals distributions. For example, reference is made to application Ser. No. 082,805, supra, which discloses a process wherein the ratio and proportion of rhenium relative to platinum is modified on the catalysts dispersed between the several reactors of a series to provide admirably high stability credits and higher conversions of the product to C.sub.5 + liquid naphthas. In accordance with the process, a series of reactors, each contains a bed, or beds, of a platinum-rhenium catalyst. The catalysts in the lead reactors are constituted of supported platinum and may contain relatively low concentrations of rhenium, with the catalyst in the last reactor of the series being constituted of platinum and a relatively high concentration of rhenium, the amount of rhenium relative to the platinum in the last reactor being present in an atomic ratio of at least about 1.5:1 and higher, or preferably 2:1, and higher. In its preferred aspects, the lead reactors of the series are provided with platinum-rhenium catalysts wherein the atomic ratio of the rhenium:platinum ranges from about 0.1:1 to about 1:1, preferably from about 0.3:1 to about 1:1, and the last reactor of the series is provided with a platinum-rhenium catalyst wherein the atomic ratio of the rhenium:platinum ranges from about 1.5:1 to about 3:1, or preferably from about 2:1 to about 3:1.
In a reforming operation, one or a series of reactors, or a series of reaction zones, are employed. Typically, a series of reactors are employed, e.g., three or four reactors, these constituting the heart of the reforming unit. It was known, and described in the '805 application, that the amount of coke produced in an operating run increased progressively from a leading reactor to a subsequent reactor, or from the first reactor to the last reactor of the series as a consequence of the different types of reactions that predominate in the several different reactors. The sum-total of the reforming reactions, supra, occurs as a continuum between the first and last reactor of the series, i.e., as the feed enters and passes over the first fixed catalyst bed of the first reactor and exits from the last fixed catalyst bed of the last reactor of the series. The reactions which predominate between the several reactors differ dependent principally upon the nature of the feed, and the temperature employed within the individual reactors. In the initial reaction zone, or first reactor, which is maintained at a relatively low temperature, the primary reaction involves the dehydrogenation of naphthenes to produce aromatics. The isomerization of naphthenes, notably C.sub.5 + and C.sub.6 naphthenes, also occurs to a considerable extent. Most of the other reforming reactions also occur, but only to a lesser, or smaller extent. There is relatively little hydrocracking, and very little olefin or paraffin dehydrocyclization occurs in the first reactor. Within the intermediate reactor zone(s) or reactor(s), the temperature is maintained somewhat higher than in the first, or lead reactor of the series, and the primary reactions in the intermediate reactor, or reactors, involve the isomerization of naphthenes and paraffins. Where, e.g., there are two reactors disposed between the first and last reactor of the series, the principal reaction involves the isomerization of naphthenes, normal paraffins and isoparaffins. Some dehydrogenation of naphthenes may, and usually does occur, at least within the first of the intermediate reactors. There is usually some hydrocracking, at least more than in the lead reactor of the series, and there is more olefin and paraffin dehydrocyclization. The third reactor of the series, or second intermediate reactor, is generally operated at a somewhat higher temperature than the second reactor of the series. The naphthene and paraffin isomerization reactions continues as the primary reaction in this reactor, but there is very little naphthene dehydrogenation. There is a further increase in paraffin dehydrocyclization, and more hydrocracking. In the final reaction zone, or final reactor, which is operated at the highest temperature of the series, paraffin dehydrocyclization, particularly the dehydrocyclization of the short chain, notably C.sub.6 and C.sub.7 paraffins, is the primary reaction. The isomerization reactions continue, and there is more hydrocracking in this reactor than in any of the other reactors of the series.
It was also generally known that the increased levels of coke in the several reactors of the series caused considerable deactivation of the catalysts. Whereas the relationship between coke formation, and rhenium promotion to increase catalyst selectivity was not, and is not known with any degree of certainty because of the extreme complexity of these reactions, it was, and is believed that the presence of the rhenium minimizes the adverse consequences of the increased coke levels, albeit it does not appear to minimize coke formation in any absolute sense. Accordingly, in the invention described by the '805 application, supra, the concentration of the rhenium was progressively increased in those reactors where coke formation is the greatest, but most particularly in the last reactor of the series to counteract the normal effects of coking.
Subsequent, it was found and disclosed in our parent application Ser. No. 336,495, supra, that yet higher activity and yield credits could be obtained by the more extensive use of a high rhenium, rhenium promoted platinum catalyst. In such process, the high rhenium, rhenium promoted platinum catalyst constituted at least one-half, and preferably constituted from about 50 percent to about 90 percent, of the total catalyst charged to the several reactors of a reforming unit, the catalyst being concentrated within at least the final reactors, or reaction zones, of the series, whereas at least about 10 percent of the forwardmost reactor volume contained a platinum, or low rhenium platinum-rhenium, catalyst; this being the reaction zone wherein naphthene dehydrogenation is the principal reaction. Higher activity and C.sub.5 + liquid yields are obtained than is obtained when using catalyst systems where the high rhenium catalyst constitutes less than fifty percent of total catalyst charge as disclosed in the '805 application, supra.
These variations, and modifications have generally resulted in improving the basic process with respect to increased process stability, and increased C.sub.5 + liquid yields.
It is, nonetheless, an objective of this invention to provide a further improved process, particularly a process capable of achieving yet further improved catalyst stability, activity, and higher conversions of feed naphthas to C.sub.5 + liquids, especially at high severities, as contrasted with prior art processes.
These objects and others are achieved in accordance with the present invention embodying improvements in a process of operating a semi-regenerative or semi-cyclic reforming unit, wherein at least about 30 percent, and preferably at least about 40 percent of the total weight of catalyst charge to the reactors is a high rhenium, platinum rhenium catalyst concentrated within the most rearward reactors (or reaction zones) of the series, or in all of the reactors (reaction zones) of the series; a high rhenium platinum catalyst in accordance with this invention being one wherein the rhenium is present relative to the platinum in weight concentration of at least about 1.5:1, and preferably from about 2:1, and higher. The catalyst bed, or beds, of the forwardmost reactor (reaction zone) of the series can contain, and preferably contains a platinum catalyst, or a low rhenium, rhenium promoted platinum catalyst, or catalyst which contains rhenium in concentration providing a weight ratio of rhenium:platinum of up to about 1.2:1, and preferably up to about 1:1. In accordance with this invention, all of the several reactors (or reaction zones) of the unit are operated at temperatures ranging from about 850.degree. F. to about 950.degree. F. (Equivalent Isothermal Temperature, E.I.T.), preferably from about 900.degree. F. to about 930.degree. F. E.I.T., at pressures ranging from about 150 psig to about 350 psig, preferably from about 175 psig to about 275 psig, while the gas rate is maintained at from about 2500 to about 4500 SCF/B, preferably from about 3000 to about 4000 SCF/B. Optimum yield credits for the staged rhenium system of this invention, it is found, are obtained by operating at severities which will provide a run length of at least 700 hours, and preferably a cycle length ranging from about 700 hours to about 2750 hours. Yield credits for the staged rhenium system of this invention are smaller at severities which result in cycle lengths less than about 700 hours or more than about 2750 hours.
The present invention requires the use of a high rhenium, platinum-rhenium catalyst within the reforming zone wherein the primary, or predominant reactions involves the dehydrocyclization of paraffins and olefins. Preferably, it also includes the use of a high rhenium, platinum-rhenium catalyst in the zone, or zones wherein the primary, or predominant reactors involves the isomerization of naphthenes, normal paraffins and isoparaffins. A high rhenium, platinum-rhenium catalyst can also be employed in the zone wherein naphthene dehydrogenation is the primary, or predominant reaction. Within the paraffin dehydrocyclization zone, and more preferably with both the paraffin dehydrocyclization and isomerization zones, there is employed a platinum-rhenium catalyst which contains rhenium in concentration sufficient to provide a weight ratio of rhenium-platinum of at least about 1.5:1, and higher, preferably at least about 2:1, and higher, and more preferably from about 2:1 to about 3:1. The naphthene dehydrogenation zone constitutes the lead zone of the series. The zone, or zones wherein the isomerization reactions predominate follows the zone wherein naphthene dehydrogenation is the primary, or dominant reaction. The naphthene dehydrogenation zone is found in the first reactor where a series of reactors constitutes the reforming unit. The isomerization zone, or zones, where a series of reactors constitute the reforming unit, may be found at the exit side of the first or lead reactor, and generally in the intermediate reactor, or reactors, of the series, or both. The paraffin dehydrocyclization zone, where a series of reactors constitute the reforming unit, is found in the last reactor, or final reactor of the series. Of course, where there is only a single reactor, quite obviously the isomerization reactions will predominate in the bed, or beds, defining the zone following that wherein naphthene dehydrogenation is the primary reaction. The paraffin dehydrocyclization reaction will predominate in the catalyst bed, or beds, defining the next zone downstream of the isomerization zone, or zone located at the product exit side of the reactor. Where there are multiple reactors, quite obviously the paraffin dehydrocyclization reaction will predominate in the catalyst bed, or beds defining a zone located at the product exit side of the last reactor of the series. Often the paraffin dehydrocyclization reaction is predominant of the sum-total of the reactions which occur within the catalyst bed, or beds constituting the last reactor of the series dependent upon the temperature and amount of catalyst that is employed in the final reactor vis-a-vis the total catalyst contained in the several reactors, and temperatures maintained in the other reactors of the reforming unit.
In its preferred aspects, the forwardmost reactor (reaction zone) of the reforming unit can contain up to about 20 weight percent, and preferably at least about 10 weight percent, of an unpromoted platinum catalyst, or a low rhenium, rhenium promoted platinum catalyst, with the remainder of the catalyst of the unit being constituted of a high rhenium, rhenium promoted platinum catalyst. Conversely, the rearwardmost reactor, or reactors, of the reforming unit will contain at least 30 percent, preferably from about 40 percent of about 90 percent of the total weight of catalyst charge in the reactors, as a high rhenium, rhenium promoted platinum catalyst. It has been found, with such loadings of high rhenium, platinum rhenium catalysts, that catalyst stability, catalyst activity, and C.sub.5 + liquid yield are a function of operating conditions, especially as relates to reactor pressure and recycle gas rate. These operating conditions with such catalyst loadings will provide, over the total length of an operating run, maximum catalyst stability, catalyst activity and C.sub.5 + liquid yield at good product octane levels.
It was found by Swan and Oyekan, supra, that staging rhenium promoted platinum catalysts in the several reactors of a reforming unit based on rhenium concentration, particularly the placement of high rhenium, rhenium promoted platinum catalysts in the final reactor of a series, which represents a maximum of 30 to 40 percent concentration of the high rhenium, platinum rhenium catalyst within the total reactor space, provided increased catalyst activity and yield credits relative to the use of the more conventional rhenium stabilized platinum catalyst in the several reactors of the unit. Then, quite surprisingly, we found and disclosed in our copending application Ser. No. 336,495, supra, that yet considerably higher catalyst activity and yield credits could be obtained by the more extensive use, in a moderately severe semi-regenerative or semi-cyclic reforming operation, of a high rhenium, rhenium promoted platinum catalyst, i.e., one wherein the high rhenium, rhenium promoted platinum catalyst constituted at least one-half, and preferably from about 50 percent to about 90 percent, of the total catalyst charged to the several reactors of a reforming unit, said catalyst being concentrated within at least the final reactors, or reaction zones, of the series. It was found, e.g., that the yield credit was increased from about 0.5 to 1 liquid volume percent (LV%) to about 2 to 3 LV% in a moderate severity semi-regenerative reforming operation, as contrasted with a similar operation at higher severity such as is typical in cyclic operations. Thus, the use of a high rhenium, rhenium promoted platinum catalyst in the final reactor of a series of reactors as employed in a cyclic reforming operation by Swan and Oyekan provided yield credits of at least about 0.5 to +1 LV%, as contrasted with conventional operations which utilized platinum-rhenium catalysts having a rhenium:platinum of up to about 1:1 in all of the reactors of a unit. Then, as disclosed in our '495 application, we found that yields in semi-regenerative operations could be further significantly improved by charging at least fifty percent of the total reactor space with high rhenium, rhenium promoted platinum catalyst. Now, we have found that these yield advantages can be further increased. Catalyst stability, catalyst activity, and C.sub.5 + liquid yield credits for staged rhenium systems in moderately severe semi-regen or semi-cyclic conditions are quite surprisingly much higher, i.e., 2-3 LV%, than in either lower severity semi-regen or higher severity cyclic conditions. Albeit the reasons for these advantages are not understood, certain observations and conclusions can be made.
Rhenium reduces the adverse affects of coke on the catalyst particularly, it is believed, as relates to the use of high concentrations of rhenium relative to the platinum hydrogenation-dehydrogenation component. The rate of coke build-up on the catalyst increases with increasing temperature, reduced hydrogen pressure, and increased oil partial pressure. Conversely, the rate of coke build-up, or catalyst deactivation, is slowest at high pressure semi-regenerative conditions, high pressure/high recycle gas rates, and low temperatures. The rate of coke build-up therefore is most rapid at cyclic reforming conditions which requires low pressures, low gas rates and high temperatures. Intermediate severity "semi-cyclic" operations typically combine the low pressures and gas rates of cyclic operations with the lower temperatures of semi-regen operations and hence the coke effect in semi-cyclic operations is intermediate those of semi-regenerative and cyclic operations. It is thus postuated in accordance with this invention that the nature of coke and its affect on catalyst selectivity is affected by process conditions. Yield credits for the high rhenium, platinum rhenium catalysts at intermediate severities ("semi-cyclic" or low pressure semi-regenerative conditions) are higher than those credits obtained with the same catalysts at low severity semi-regenerative conditions because increased coking rates occur at semi-cyclic conditions. However, as severity is increased further as in moving from semi-cyclic to cyclic conditions, coke properties change in such a way that the yield credits for high-rhenium catalysts are reduced.
It was exemplified in application Ser. No. 082,805, supra, that yield and activity credits could be obtained by charging the final reactor of a multi reactor reforming unit with a high rhenium, rhenium promoted platinum catalyst, and the lead and intermediate reactors with a more conventional platinum-rhenium catalyst wherein the rhenium:platinum ratio of the catalyst approximated 1:1. These credits were demonstrated at relatively low pressure cyclic conditions (175 psig, 3000 SCF/B, 950.degree. F. E.I.T., relatively high pressure semi-regenerative conditions (400 psig, 6000 SCF/B, around 900.degree. F. start-of-run (SOR) temperature) and relatively high pressure "semi-cyclic" conditions (425 psig, 2500 SCF/B, approximately 900.degree. F. SOR temperature ). In each case these credits were about +0.5 to +1 LV% C.sub.5.sup.+ liquid yield and +5 to 15% initial activity for staged systems comprising 30-40% of a high rhenium, platinum rhenium catalyst in the final reactor of the series, as contrasted with a conventional operation. It was then exemplified in application Ser. No. 336,495, supra, that yield credits could then be further improved, i.e., by an additional 0.5 to 1 LV% C.sub.5.sup.+ liquid yield, with further increased catalyst activity in operations wherein the concentration of high rhenium, platinum rhenium catalyst were further increased to 50 percent, or 50 percent to 90 percent based on the total catalyst charged to the reforming unit. Now, it has been found that these credits can be further increased, in fact essentially doubled over those presented in the '805 application by operation at low pressure, semiregenerate or semi-cyclic conditions, while retaining the high concentration of high rhenium, platinum rhenium catalysts within the rearwardmost reactors of the unit.
The following data, by way of comparison, was presented in the '805 application. All parts are in terms of weight units except as otherwise specified. These data are demonstrative of the activity and yield advantages obtained at high severity cyclic conditions by the use of a high rhenium platinum-rhenium catalyst in the tail reactor of a multiple reactor unit reformer, with a low rhenium, platinum-rhenium catalyst in the several lead reactors, to wit: